1. Field of the Invention
The present invention relates to a control unit of a vehicle generator, and more particularly to such a control unit of a vehicle generator capable of instantaneously stopping power generation of the vehicle generator in a safe manner when a key switch 4 of a vehicle is turned off.
2. Description of the Related Art
FIG. 5 is a circuit diagram illustrating the configuration of a known control unit of a vehicle generator which is capable of instantaneously stopping power generation of the vehicle generator upon turning off of a key switch 4 of a vehicle.
In FIG. 5, the known control unit includes a control circuit 1 for controlling the generation voltage of a vehicle generator 2, which is driven by an engine (not shown), at a predetermined voltage, a charge lamp 3, a key switch 4 of a vehicle, a battery 5, and a power generation stop circuit 6 for stopping the power generation of the vehicle generator 2 when the key switch 4 is turned off.
Here, the vehicle generator 2 has a field coil 2a, a three-phase stator coil 2b, a three-phase full wave rectifier 2c for rectifying the generation voltage of the three-phase stator coil 2b, and an auxiliary rectifier 2d for supplying a field current. A direct current full-wave rectified by the three-phase full wave rectifier 2c is supplied to the battery 5.
In addition, the control circuit 1 includes a combination of a diode 1a and voltage dividing resistors 1b, 1c connected between a voltage sensing terminal S and ground for detecting the voltage of the battery 5, a Zener diode 1d having a cathode connected with a connection point or node between the voltage dividing resistors 1b and 1c, a combination of a resistor 1e and a diode 1f provided between the resistor 1b and an output terminal L, a combination of a resistor 1g and an emitter-grounded npn transistor 1h provided between the output terminal L and ground, a flywheel diode 1i similarly provided between the output terminal L and ground, and a transistor 1j with a Darlington connection for turning on and off the field current. The Zener diode 1d as an anode connected with a control terminal of the transistor 1h. The control terminal of the transistor 1j is in turn connected with a connection point or node between the resistor 1g and the collector of the transistor 1h. 
The power generation stop circuit 6 includes an off detection circuit 6a for detecting the turning off of the key switch 4, a transistor 6b which is made conductive based on an off detection signal of the off detection circuit 6a, a power generation stop relay 6c which is energized to close its contacts thereby to ground the output terminal L when the transistor 6b is made conductive, and a diode 6d provided between the key switch 4 and the output terminal L.
The known control unit of a vehicle generator as constructed above operates as follows.
The control circuit 1 detects the voltage of the battery 5 through the voltage sensing terminal S, and when the voltage of the battery 5 rises above a predetermined value which has been set in advance, that is, when the voltage imposed on the cathode of the Zener diode 1d rises above a Zener voltage, the transistor 1h is made conductive and the transistor 1j is made nonconductive so that a field current supplied to the field coil 2a is decreased, thus reducing the generation voltage of the generator 2. On the contrary, when the voltage of the battery 5 lowers, the transistor 1h is made nonconductive and the transistor 1j is made conductive whereby the field current is increased, thus raising the generation voltage of the generator 2.
When the key switch 4 is turned off in order to stop the engine of the vehicle which is now in operation, the off detection circuit 6a for the key switch 4 detects the turning off of the key switch 4 and makes the transistor 6b into a conductive state, whereby the contacts of the power generation stop relay 6c is turned on, thus grounding the output terminal L of the auxiliary rectifier 204 which supplies a field current to the field coil 2a. 
As a result, current does not flow to the field coil 2a and hence power generation is stopped instantaneously. Thus, it becomes possible to stop power generation regardless of the operating conditions of the engine of the vehicle (even if the engine is coasting or rotating under the inertia effect).
In the known control unit of a vehicle generator as constructed above, in order to stop generation of electric power instantaneously, when the key switch 4 is turned off, the relay 6c is energized to ground the output terminal L of the auxiliary rectifier 2d thereby to interrupt the field current. For this reason, at the instant when the output terminal L of the auxiliary rectifier 2d is grounded, a large current in excess of 100 amperes might flow to the auxiliary rectifier (excitation diode) 2d, depending upon the power generation state of the generator 2 (the state of an electric load of the vehicle). Thus, there arises a problem that the auxiliary rectifier (excitation diode) 2d is required to have a large capacity.
Moreover, since a large current might flow through the relay 6c, too, the relay 6c is also required to have a large current-carrying capacity. In addition, there is another problem that a large current flowing through the relay 6c generates sparks at its contacts, which is dangerous.
In case of using a mechanical relay, there arise further problems such as wear of contacts, increased contact drop, etc., thus shortening the lifetime of the vehicle system.